What is the difference between a small and large diaphragm in a condenser mic?
Here is an admittedly basic explanation:
Small diaphragm condenser mics tend to be "more accurate" in reproducing the acoustic signal and are often used for measurement mics, instrument mics, and recording mics for ensembles.
Large diaphragm condenser mics tend to be "less accurate" in reproducing the acoustic signal, but produce warm and unique signals that enhance solo vocals or small vocal ensembles.
However, there is no rule that says a small diaphragm mic cannot be used for vocals or a large diaphragm mic for instruments. Use the mic that produces the sound quality you are seeking.